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Signal tone
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<p>A signal tone or signalling tone is a steady or pulsating <a href="/facts/Periodic_function/Sm8lJQsF">periodic</a> signal typically in the frequency range of <a href="/facts/Sound/DlKXlPuF">sound</a> for indicating a condition, <a href="/facts/Communication_protocol/5uady7cB">communication protocol</a> state, or serve as an audible warning. It may be composed of multiple frequency components, or could be a <a href="/facts/Pure_tone/E2FyqF8k">pure tone</a>. </p><p>In telephone systems, signaling tones are used as <a href="/facts/Call_progress_tone/BXQ8Lonr">call progress tones</a> for <a href="/facts/In-band/KAgeV09X">in-band</a> indications to subscribers or operators. Certain telephone switching systems used tones, in-band or out-of-band, for <a href="/facts/Signaling_(telecommunications)/6d4zgUj7">signaling</a> on trunks. </p><p>Typical well-known call progress tones are <a href="/facts/Dial_tone/jA21wRp5">dial tone</a>, <a href="/facts/Ringing_tone/s3elA86F">ringing tone</a>, <a href="/facts/Busy_tone/69RRCIA7">busy tone</a>, and the <a href="/facts/Reorder_tone/vo0VU9RV">reorder tone</a>. A loud stutter tone is used to alert subscribers of a handset left off-hook, effectively disabling the circuit for receiving calls. </p><p>Telephone service subscribers may subscribe to services, such as call forwarding, which may indicate function by a stutter dial tone. </p>